Concurrency: Mutual Exclusion and Synchronization

• Why we need Mutual Exclusion?

• Classical examples:

• Bank Transactions:Read Account (A); Read Account (A);

Compute A = A + 1000; Compute A = A – 500;

Write Account (A); Write Account (A);

• Keep A = B all the timeA = A + 1; A = 2 * A;

B = B + 1; B = 2 * B;

• Wrong results if mutual exclusion is not enforced!

Disable InterruptSoftware Solution•Dekker’s Algorithm•Peterson’s Algorithm

Hardware Solution•Disable Interrupt•Test & Set•Exchange Instruction

Monitors

• Semaphores is a powerful and flexible tool for enforcing mutual exclusion and for coordinating processes.

• It may be difficult to produce a correct program and very difficult to debug.

• The difficulty is that wait and signal operations will be scattered throughout a program and it is not easy to see the overall effect of these operations on the semaphores they affect.

• A monitor is a programming language construct that provides equivalent functionality to that of semaphores but easier to control.

• Monitors is implemented in:– Concurrent Pascal

– Pascal-Plus

– Modula-2, Module-3

– Ada

– As a programming library

Monitors (continue)• Monitors can lock on any object.

• One may want to lock all linked list with one lock;

• or lock a particular linked list by one lock;

• or lock an element of a particular linked list by one lock.

Monitor with Signal– cwait(condition): Suspended execution of the calling process on a condition. The

monitor is now available for use by another process.

– csignal(condition): Resume execution of some process suspended after a cwait on the same condition. If there are several such processes, choose one of them; if there is no such process, do nothing.

• A monitor is a software module consisting of one or more procedures, an initialization sequence, and local data.

– The local data variables are accessible only by the monitor’s procedures and not by any external procedures.

– A process enters the monitor by invoking one of its procedures.

– Only one process may be executing in the monitor at a time; any other process that has invoked the monitor is suspended, waiting for the monitor to become available.

Message Passing• Two fundamental requirements must be satisfied when processes interact: synchronization and

communication.

• Message passing is a common approach for distributed systems.

• It is normally provided in the form of a pair of primitives:

– send(destination, message)

– receive(source, message)

• Major design characteristics of message systems for Inter-processor Communication and Synchronization:

– Synchronization

• Send : blocking vs. non-blocking

• Receive : blocking, non-blocking, test for arrival

– Addressing:

• Direct

– Send

– Receive : explicit vs. implicit

• Indirect

– Static, dynamic, ownership

– Format: message content; Length -- fixed vs. variable

– Queuing Discipline: FIFO, Priority Driven

Readers/Writers Problem

The readers/writers problem is defined as follows:

• There is a data shared among a number of processes.

• The data area could be a file, a block of main memory, or even a back of processor registers.

• There are a number of processes that only read the data (readers) and a number of processes that only write to the data (writers)

• The following conditions must be satisfied:

– 1. Any number of readers may read the data at the same time.

– 2. Only one writer at a time may write to the data.

– 3. If a writer is writing to the data, no reader may read it.

• Two observations:

– Readers have priority (writers might starve)

– Writers have priority